Characterization of adenosine receptor(s) involved in adenosine-induced bronchoconstriction in an allergic mouse model

We recently reported that adenosine caused bronchoconstriction and enhanced airway inflammation in an allergic mouse model. In this study, we further report the characterization of the subtype of adenosine receptor(s) involved in bronchoconstriction. 5'-(N-ethylcarboxamido) adenosine (NECA), a nonselective adenosine agonist, elicited bronchoconstriction in a dose-dependent manner. Little effects of N-6-cyclopentyladenosine (A(1)-selective agonist) and 2-p-(2-carboxyethyl) phenethylamino-5'-N-ethylcarboxamidoadenosine ( A(2A)-selective agonist) compared with NECA were observed in this model. 2-Chloro-N-6-(3-iodobenzyl)9-[ 5-(methylcarbamoyl)-beta-D-ribofuranosyl] adenosine, an A(3)-selective receptor agonist, produced a dose-dependent bronchoconstrictor response, which was blocked by selective A(3) antagonist 2,3-diethyl-4,5-dipropyl-6-phenylpyridine-3-thiocarboxylate-5- carboxylate (MRS1523). However, MRS1523 only partially inhibited NECA-induced bronchoconstriction. Neither selective A(1) nor A(2A) antagonists affected NECA-induced bronchoconstriction. Enprofylline, a relatively selective A(2B) receptor antagonist, blocked partly NECA-induced bronchoconstriction. Furthermore, a combination of enprofylline and MRS1523 completely abolished NECA-induced bronchoconstrictor response. Using RT-PCR, we found that all four adenosine receptor subtypes are expressed in control lungs. Allergen sensitization and challenge significantly increased transcript levels of the A(2B) and A(3) receptors, whereas the A(1) receptor message decreased. No change in transcript levels of A(2A) receptors was observed after allergen sensitization and challenge. These findings suggest that A(2B) and A(3) adenosine receptors play an important role in adenosine-induced bronchoconstriction in our allergic mouse model. Finally, whether the airway effects of the receptor agonists/antagonists are direct or indirect needs further investigations.